1. Field of the Invention
The present invention relates to a method of producing an aluminum base alloy containing silicon.
2. Description of the Background Art
An aluminum base alloy containing silicon is generally produced by ingot metallurgy (I/M) method of adding and fusing silicon into a molten aluminum alloy, a pre-mixed powder extrusion method of mixing silicon particles with aluminum alloy particles and extruding into a shape the pre-mixed powder via a powder metallurgy method.
In the ingot metallurgy (I/M) method, however, larger primary crystals of silicon are crystallized and segregated in the aluminum base alloy, whose strength is reduced and machinability is deteriorated as the result. In the pre-mixed powder extrusion method, on the other hand, the aluminum alloy particles and the silicon metal particles are insufficiently joined at the interfaces therebetween, and hence the as-formed aluminum base alloy is inferior in strength and toughness. Further, less stress is transferred due to insufficient bonding at the interfaces, and hence reduction in the thermal expansion coefficient is less than that of expected.
Such problems can be solved by a powder extrusion method of pre-alloy powder in which pre-alloy powder is prepared via atomizing a molten Al-Si alloy, and extruding into a shape. According to this method, it is possible to obtain an aluminum base alloy which has high strength, toughness and machinability and a low thermal expansion coefficient.
However, when an aluminum base alloy containing high concentration of silicon is produced by such an alloy powder extrusion method, the melting temperature of the aluminum alloy is raised up as the silicon content is increased, and hence it is necessary to heat the aluminum alloy to a high temperature in order to melt the same. When alloy powder is prepared by the atomization method, therefore, a problem such as plugging of a nozzle is caused during atomization, leading to problems in productability and economics.
Also when solid particles such as graphite particles are dispersed in an aluminum base alloy containing silicon, the conventional I/M method and powder metallurgy (P/M) method cause the following problems:
In the I/M method, solid particles which have different specific gravity from the molten alloy are added into the molten alloy. Thus, the solid particles are segregated in the molten alloy due to the difference in specific gravity, and hence it is impossible to homogeneously disperse the solid particles in the aluminum base alloy. In order to solve such a problem, proposed is a method of plating the surfaces of graphite particles with nickel and mixing with a molten alloy, for example. If graphite particles are thus plated, however, the cost is significantly increased to cause a problem in economics. In addition, matrix strength is reduced due to a slow solidification rate. When bare graphite particles are employed, further, Al.sub.4 C.sub.3 is generated at interfaces between the particles and the matrix of the aluminum alloy, to reduce toughness of the as-formed aluminum base alloy.
In the P/M method, graphite particles and aluminum alloy particles are mixed up and then consolidated. Thus, bonding strength between the graphite particles and the matrix is made insufficient, leading to reduction in strength and toughness of the aluminum base alloy. Further, the graphite particles are deformed into flaky shape by shear breakage layer by layer during plastic working, to reduce bonding strength between aluminum alloy particles.